Notes:1.
It is what antonyms were called before the word antonym was coined in about 1870.

Example -
The counterterm of the word up is down.

Oxford English Dictionary - The term's first citation is from 1870: "The Etymology of the word … a merely expresses the idea of one word in substitution for, which in matters of verbal debate, is equivalent practically to opposition to another; a double force which, in addition to its analogy to Synonym, seemed to render Antonym a preferable word to Counterterm."(C. J. Smith Syn. & Antonyms Pref. 6)